Conventional alkali metal ionization detectors, such as those used to monitor sodium, potassium, lithium, etc., operate in vacuum and employ pure metal vacuum filaments and strive to minimize or avoid surface oxidation which has been considered to be detrimental to the useful operation of the detector filaments. Typical vacuum filament material used in alkali metal ionization detectors includes thoria-coated iridium, tungsten, platinum and rhodium. In alkali metal ionization detectors, such as that disclosed in U.S. Patent application Ser. No. 435,389, filed Jan. 21, 1974, entitled "Sensor For Thermally Ionizable Particles And/Or Vapors," which is assigned to the assignee of the present invention and incorporated herein by reference, the detector operating principle consists essentially of thermally ionizing particles or vapors contacting a heated filament to produce ions which are attracted to a collector electrode via an electric field thus producing a current flow which is an indication of the concentration of particles or vapors from which the ions were formed. One of the major considerations in selecting the material composition for the heated filament for conventional alkali metal ionization detectors has been the material's capability of serving as an electron conductor free of surface oxidation which would form an insulator coating thereby ostensibly reducing the effectiveness of the filament. While standard vacuum filament materials such as tungsten, platinum and platinum-rhodium, have operated satisfactorily in vacuum-type ionization detectors, the use of these conventional filament materials in alkali metal ionization detectors operating in environments containing oxygen and at pressures above vacuum, such as atmospheric pressure, have proven unsatisfactory due to the drastic reduction in filament operating life.
There is described herein with reference to the accompanying drawings the use of heating element material as a filament in an alkali metal ionization detector which to produce an alkali metal ionization detector capable of operating satisfactorily over an extended period of time in both an oxygen environment at pressures above or below atmospheric pressure, and a vacuum environment. This eliminates the critical requirement for the vacuum conditions of conventional alkali metal ionization detectors. An oxide coating is developed at the surface of the heating element material to produce a useful filament capable of supporting surface ionization and electron conductivity and having an operating life exceeding that offered by conventional filament materials.